| At present,the world is facing the problem of resource shortage and environmental pollution,and wind power is rapidly developing with the advantages of clean and renewability.The Brushless Doubly-Fed Induction Generator(BDFIG)operates in a similar way to the Doubly-Fed Induction Generator(DFIG)that is widely used in wind power systems,and both have the advantages as the DFIG.However,the BDFIG,without carbon brushes and slip rings,improves the operational stability and reduces the costs of operation and maintenance,thus having a broad application prospect.After nearly a decade of research,the BDFIG application technology is increasingly maturing and taking another step towards marketability.With the increasing development of wind power,many requirements about the operation of wind power systems need to be met,such as low-voltage ride-through,high-voltage ridethrough capability,and the power system power quality improvement after the wind power system is connected to the grid.The study focuses on the control strategies of BDFIG in grid voltage unbalance and the most severe asymmetric low voltage ride-through of BDFIG based on the magnetic chain tracking control strategy.And the specific contents is as follows:1.The improved indirect power control strategy for brushless doubly-fed induction generators during grid voltage imbalanceThe study derives the equations of active as well as reactive power of BDFIG under the allowed imbalance of the grid and analyze the effects caused by the grid imbalance on BDFIG.Based on the above,the study proposes an improved control strategy of indirect power control for BDFIG grid imbalance and constructs a structural block diagram of the control system.Simulation results in MATLAB/Simulink environment show that,compared with the relevant existing literature,the advantage of the proposed control strategy of the study is that the pulsation values of the output active and reactive power caused by the negative sequence voltage components are simultaneously taken as the control objectives,and the pulsation of the active and reactive power are simultaneously reduced to an acceptable level.2.Research on the most severe asymmetric low-voltage fault ride-through method of BDFIG based on magnetic chain tracking controlThe results of the paper show that the severity of BDFIG asymmetric LVRT(that is to say,the magnitude of the BDFIG control winding current during fault ride-through)is closely related to the phase of the grid voltage at the time of the fault.A single-phase grounded short circuit occurs at different grid voltage phases,and the maximum control windi ng current is2.96 times the minimum current.However,at present,both domestic and foreign countries do not consider the influence of the moment of fault occurrence when verifying the feasibility of BDFIG asymmetric LVRT control strategy and evaluating the size of the control winding current,thus resulting in the evaluation results losing their meaning.For the problem that the BDFIG mathematical model is high-order,nonlinear,strongly coupled,and unable to derive the analytical expression of the control winding current,the study proposes a method that combines theoretical analysis and numerical calculation to solve the most severe moments of asymmetric faults(single-phase ground fault,two-phase ground fault,and two-phase phase-to-phase short-circuit fault),based on the low-voltage ridethrough control strategy of magnetic chain tracking control.In the study,the expressions of the steady-state currents of the control winding before and after the fault are obtained,and then the expressions of the control winding currents after the fault are obtained according to the principle that the control winding currents cannot change abruptly during the fault and after considering the decay time constants.Last the most severe moments of the asymmetric fault are obtained using numerical calculations.The simulation results in MATLAB/Simulink environment basically match with the numerical calculation results,which verifies the feasibility of the proposed method of the study and also verifies that the chain tracking LVRT control strategy can control the control winding current within the allowed range even under the most severe fault conditions.Meanwhile,the study analyzed other factors influencing the magnitude of the control winding current asymmetric faults,and results showed that the rated load,maximum speed,and the largest value of control winding current inrush when an asymmetric fault occurs.3.Development and implementation of asymmetric voltage dip generator based on AC non-contact switchBecause the severity of BDFIG asymmetric LVRT is related to the moment of fault occurrence,the voltage drop device developed by the current laboratory uses a PLC-controlled contactor as a switch,which cannot accurately realize the asymmetric voltage drop at the most severe moment due to its action time of ms level.The author developed an asymmetric voltage dip generator based on contactless switch.During the process,the structure of contactless switch is determined,IGBT is selected as the switching device,and the absorption circuit is determined to ensure the normal operation of the switching device.The simulation model is built in MATLAB/Simulink environment,which verifies the feasibility of the block diagram of the main circuit of the asymmetric voltage di p generator.The experimental results demonstrate that the constructed experimental setup can be used with the main control cabinet of BDFIG to perform the verification of the most severe moments of asymmetric voltage dips based on magnetic chain tracking.4.Low voltage ride-through experiment for the most severe single-phase ground faultOn the experimental platform of 15 k W brushless doubly-fed induction generator newly built by the group,the study completed the experiment of low voltage fault ride-through in the most severe case of single relative midpoint based on the magnetic chain tracking control and designed the experimental method.Since there was a discrepancy between the motor parameters used in the theoretical calculation and the experimental prototype,the study used the experimental data and calculate the most severe moments by utilizing a combination of the theoretical analysis and numerical calculations proposed in Research Element 2.And six moments were taken within 0-24 ms near the most severe moment,and single-phase relative midpoint low voltage fault ride-through experiments(rated load,maximum speed)were conducted.The experimental results show that the most severe moments basically match the calculated results,with the maximum and minimum control winding currents of 2.12 p.u.and0.717 p.u.,respectively,the former being 2.96 times higher than the latter.At the same time,the experimental results show that the LVRT control method of chain tracking control can still limit the control winding current to 2.12 p.u.even under the most severe fault conditions. |
PDF Full Text Request